In U.S. Pat. No. 4,457,664, Judell et al. disclose a wafer alignment station having a capacitive sensor producing an electrical signal representative of the position of the wafer edge along an X axis. The wafer is spun on a vacuum chuck one full turn and the signal is sampled to produce data for a number of points around the wafer. A processor filters out data corresponding to noise and flat edge information, then calculates an eccentricity vector defined between the centroid of the wafer and the point of rotation about the axis of the vacuum chuck. The wafer is then centered on the chuck, and spun a second full turn. Data obtained from this second spin is used to calculate the position of the flat edge region relative to the X axis, and also serves as a check on whether or not the wafer is aligned about its centroid. The wafer is then spun to the desired orientation.
The calculation of the eccentricity vector is accomplished by fitting the filtered data to an equation, derived from a model of predicted observations, in a least squares fit with the X and Y components of the vector and the radius of the wafer as unknowns. Upon solving for the unknowns by means of determinants, the magnitude and direction of the vector is derived. The calculation of the flat position is accomplished by computing a threshold equal to 3/4 of the mean value of the data plus 1/4 of the value of the minimum data point, identifying the threshold crossings of the data points, and fitting the data between these crossings to a parabola.
Optical flat edge finding apparatus are known and use various techniques to orient the wafer in a desired position. For example, in U.S. Pat. No. 4,418,467 to Iwai, alignment marks are formed on a side of the wafer. In U.S. Pat. Nos. 4,376,581 and 4,461,567 to Mayer, U.S. Pat. No. 4,539,481 to Troukens et al. and U.S. Pat. No. 3,890,508 to Sharp, photodetectors near an edge of a wafer are normally covered by the wafer except when the wafer is properly aligned. At that time, the detectors are adjacent to a flat region and being near the wafer edge are uncovered permitting light to reach them.
Unfortunately, prior art wafer aligners are slowed by the necessity of having an already centered wafer to properly locate the flat region of a wafer and thereby correctly orient the wafer. Two separate steps are thus required. A first step spins the wafer to identify the eccentricity, and the wafer is centered. Then, a second step spins the wafer again to identify the flat region of a wafer, and the wafer is oriented in a desired direction.
It is an object of the present invention to determine edge alignment in a wafer which only requires one spin step to find both the flat edge region and the wafer eccentricity.
It is another object of the invention to determine flat edge region alignment in a wafer wherein the determination does not require the wafer to be spun about its center.